Gas detector and its adjusting method

Abstract

A metal oxide semiconductor gas sensor S is connected to a ladder resistance R, and the output voltage is subjected to logarithmic transformation at plural points on a waveform of temperature change by LnR=2-4 V R1/Vc+LnR1 (1) where R indicating the resistance of the metal oxide semiconductor, V R1 the output voltage to the ladder resistance, Vc the detecting voltage, R1 the resistance of the ladder resistance, and Ln natural logarithm, respectively. Standard signals comprising logarithms of resistance values of the metal oxide semiconductor in plural concentrations and at plural points on the waveform are stored in the EEPROM, and these standard signals and logarithms obtained are compared with each other to detect the gas.

We claim: 
     
       1. A gas detector detecting a gas by subjecting a metal oxide semiconductor gas sensor to a temperature change, a resistance of said gas sensor changing with the gas, said gas detector comprising: a non-volatile memory means for storing a standard signal being linear to logarithms of resistance values of said metal oxide semiconductor in combination with plural points on a waveform of the resistance of the gas sensor according to the temperature change;   a ladder resistance being connected as a load resistance in series with said gas sensor, a ratio of its resistance to that of the metal oxide semiconductor being arranged to be within a specified range at said plural points;   a power source for applying a detecting voltage to said ladder resistance and said metal oxide semiconductor;   a sampling means for sampling measurement data being linear to the logarithm of the resistance value of said metal oxide semiconductor by sampling an output linear to an output voltage to said ladder resistance at each of said plural points; and   a gas detecting means for detecting the gas by comparing combinations of measurement data at said plural points with said standard signal.   
     
     
       2. A gas detector of claim 1 characterized in that in said sampling means, said output is transformed into said measurement data by   LnR=2-4 V R1/Vc+LnR1     where R indicating the resistance of the metal oxide semiconductor, V R1 the output voltage to the ladder resistance, Vc the detecting voltage, R1 the resistance of the ladder resistance, and Ln natural logarithm, respectively.   
     
     
       3. A gas detector of claim 1 characterized in that in said sampling means, said output is transformed into said measurement data by   LnR=2x+2x.sup.3 /3+LnR1 and       x=1-2 V R1/Vc     where R indicating the resistance of the metal oxide semiconductor, V R1 the output voltage to the ladder resistance, Vc the detecting voltage, R1 the resistance of the ladder resistance, and Ln natural logarithm, respectively.   
     
     
       4. A gas detector of claim 3 characterized in that said ladder resistance is configured so that a resistance ratio of said metal oxide semiconductor and said ladder resistance is kept within a range of from 4 to 1/4 at said plural points. 
     
     
       5. A gas detector of claim 2 characterized in that said ladder resistance is configured so that the resistance ratio of said metal oxide semiconductor and said ladder resistance is kept within a range of from 2 to 1/2 at said plural points. 
     
     
       6. A gas detector of claim 1 characterized in that for at least one point of said plural points, the resistance value of the ladder resistance is switched according to said output at the preceding point thereof. 
     
     
       7. A gas detector of claim 1 characterized in that said non-volatile memory is an EEPROM, and standard signals at least three gas concentrations are stored in the EEPROM,   that a volatile memory is provided, and out of the standard signals stored in said EEPROM, standard signals of two lower concentrations are read out of the EEPROM into said volatile memory, and   that in said gas detecting means, measurement data are compared with the standard signals stored in the volatile memory, and standard signals to be stored in the volatile memory are changed.